A method of delaminating ions-implanted wafers after bonding to manufacture SOI wafers, i.e., an ion implantation delamination method (a technique also referred to as the Smart Cut method (registered trademark)), has attracted attention as a method of manufacturing silicon on insulator (SOI) wafers, for example, particularly a method of manufacturing thin-SOI wafers that enable improvement in performance of advanced integrated circuits. This ion implantation delamination method is a technique to form an SOI wafer (See Patent Document 1) in the following manner: an oxide film is formed on at least one of two silicon wafers; gas ions such as hydrogen ions or rare gas ions are implanted from a front surface of one of the silicon wafers (a bond wafer) to form a micro bubble layer (also referred to as an enclosed layer or an ion implanted layer) in the interior of the wafer; the surface from which the ions are implanted is then brought into close contact with the other silicon wafer (a base wafer) through the oxide film; a heat treatment (a delamination heat treatment) is then performed to cleave one of the wafers (the bond wafer) along the micro bubble layer so that the bond wafer is delaminated into a thin film; and another heat treatment (a bonding heat treatment) is then performed in accordance with needs to strengthen a bond between the wafers. At this point, the cleavage plane (the delaminating plane) is a surface of an SOI layer and an SOI wafer having a thin SOI with high uniformity is relatively readily obtained.
The SOI wafer after the delamination, however, has a damage layer on its surface due to the ion implantation. This surface is rougher than a surface of a normal mirror-polished silicon wafer. The ion implantation delamination method accordingly needs to remove such a damage layer and surface roughness.
Conventionally, mirror polishing with extremely small polishing stock removal (a stock removal of about 100 nm or more), referred to as touch polishing, is performed to remove the damaged layer and so on in the final step after the bonding heat treatment. Even though the implantation of ions such as hydrogen ions and the delamination can achieve the radial uniformity of the thickness of the SOI layer (radial film thickness distribution) to a certain extent, a polishing process including such a mechanical factor performed on the SOI layer degrades this uniformity because of nonuniform polishing stock removal. Specifically, when a bonded SOI wafer is manufactured by an ion implantation delamination method using a silicon single crystal wafer with the diameter of 300 mm, for example, and flattening and removal of a damage are performed on the surface of the delaminated SOI layer only by touch polishing, the radial film thickness uniformity is inevitably degraded to ±6 nm or more after the touch polishing even when it has been ±1 nm in the SOI layer just after the delamination.
To solve this problem, a flattening process including a high-temperature heat treatment is recently performed to improve the surface roughness, instead of the touch polishing. For example, Patent Document 2 describes decreasing a film thickness of a thin film of a bonded wafer after delaminating the bond wafer by heat treatment under an atmosphere of inert gas, hydrogen gas, or mixed gas thereof, followed by thermal oxidation to form a thermal oxide film on the surface of the thin film, and removing the thermal oxide film (i.e., sacrificial oxidation process).
Another method is also described, in which a delaminated bonded wafer is subjected to heat treatment under an atmosphere of inert gas, hydrogen gas, or mixed gas thereof, followed by polishing of the surface of the thin film with a stock removal of 70 nm or less, and then a sacrificial oxidation process is performed to reduce the thickness of a thin film.